1. Field of the Invention
This invention relates to a telemetry system for transmitting analog data over a telephone line. In particular, it relates to a single channel analog telemetry system wherein an analog signal is converted to a frequency shift keyed transmission signal by way of an intermediate frequency modulated signal.
2. History of the Art
Telemetry systems for transmitting analog data over telephone lines are useful in a wide variety of applications. Such systems can be used in medicine, for example, to transmit vital medical information, such as an electrocardiogram and blood pressure to a remote medical center, and they can be used in industry to transmit process control information to a remote central process controller.
Because telephone lines introduce relatively high levels of spurious "noise" signals not properly part of the signals being transmitted, it is common practice in single channel analog telemetry systems to convert the information-containing direct current DC analog voltage or current signals to frequency shift keyed (FSK) transmission signals before transmission on the telephone lines. Typically a 0-10 volt DC signal is converted to a frequency modulated (FM) signal in the 10-30 Hertz range, and this low frequency FM signal is used to key an FSK tone transmitter which produces an FSK modulated signal for transmission over a telephone line to a remote receiver. These FSK signals--comprising modulated shifts from one discrete frequency to another--are considerably less subject to distortion by noise than are conventional AM or FM modulated signals. At the receiver, the FSK signal is demodulated to reproduce the low frequency FM signal which, in turn, is reconverted to a DC analog signal.
The thus described conventional systems suffer serious deficiencies in speed and accuracy. They are slow at the receiver and in reconstituting the analog value transmitted. This delay is due primarily to the time required to process and filter a 10-30 Hertz signal. Typically the receiver takes approximately two seconds to "settle" to approximately 98% of the analog value. This delay for each reading can be extremely expensive in control systems utilizing periodic computer monitoring and can be a source of error in metering. At the transmitter end, the system is also slow because of similar processing delays; and, in addition, is subject to errors in accuracy and linearity due to the small span of frequencies covering the range of analog values.
Accordingly, there is a need for a single channel analog telemetry system with a faster settling time at the receiver and greater linearity and accuracy at the transmitter.